The extrusion of polyurethane plastic is difficult. Polyurethane has many unique and desirable characteristics, and it is the polymer of choice for many applications. Unfortunately, polyurethane has one really serious drawback from an extrusion standpoint—it is highly variable. As remarked in the paper entitled Viscosity Control in Extrusion . . . The Viscontrol Process, TDS -298, Harrel, Incorporated, incorporated herein by reference and which is attached to the above-identified U.S. provisional application, most polymers will vary in characteristics from lot to lot, but polyurethane characteristics can vary several times in a 50-lb. sack.
Trying to extrude polyurethane on a conventional, manually controlled extruder is an exercise in frustration. Every time the polymer characteristics change, the operator has to readjust the extruder to compensate. Either that or the operator must accept the resulting changes in the dimensions of the extrudate.
For a number of years Harrel, Incorporated's TUBETROL system has been a solution to the problem of extruding polyurethane. The TUBETROL system uses a melt pump, and one of the characteristics of the melt pump is that, when properly controlled, it is capable of holding the throughput steady to 0.1% stability or so. This makes for better tubing and a considerably lower level of operator frustration.
U.S. Pat. No. 4,209,476, issued Jun. 24, 1980, entitled “Method and Apparatus for Fabrication of Extruded Articles” in the name of Holton E. Harris and assigned to the assignee of the present invention, Harrel, Incorporated, describes Harrel's TUBETROL process. It is incorporated herein by reference. In addition to a generalized explanation of maintaining cross-section dimensions of an extrudate, examples are given of the use of the system to maintain diameter of an extruded rod, thickness of an extruded sheet and inside and outside diameter, hence thickness, of an extruded tube.
Using TUBETROL to extrude urethane, operators still must manually adjust the temperatures of the barrel to adjust the viscosity of the polymer. There is a need, therefore, for an extruder control that adds viscosity control to the standard TUBETROL system. This would add a new dimension of control to the TUBETROL system, and make it into an entirely different and more capable system.
The paper, identified above, describes the viscosity control system of Harrel, Incorporated called VISCONTROL. U.S. Pat. No. 4,721,589 entitled “Extruder Viscosity Control System and Method” issued Jun. 26, 1988 to Holton E. Harris, assigned to Harrel, Incorporated, the assignee of the present invention, also describes the Harrel viscosity control system. This patent is incorporated herein by reference.
Harrel's VISCONTROL system as described in the paper and patent has never been incorporated in a Harrel TUBETROL system. Consequently, automatic viscosity control such as would benefit the extrusion of polyurethane under the dimensional control of a TUBETROL system has not previously been accomplished.